The present invention is directed toward medical electrical leads generally and more particularly directed towards medical leads employing advancable, rotating fixation helixes to anchor the leads to body tissue.
In order to work reliably, cardiac pacing leads need to be located stably adjacent tissue to be stimulated or monitored. One common mechanism for accomplishing this has been the use of a rotatable fixation helix, which exits the distal end of the lead and is screwed into body tissue. The helix itself may serve as an electrode or it may simply serve as an anchoring mechanism to locate an electrode mounted to the lead body adjacent body tissue. In conjunction with such leads, the fixation helix may be coupled to a conductor extending through the lead body and rotated by rotation of the conductor within the lead body. Such a lead is disclosed in U.S. Pat. No. 4,106,512 issued to Bisping. Alternatively, the fixation helix may be rotated by means of a screwdriver tip stylet, or other removable rotation device, as disclosed in U.S. Pat. No. 4,217,913 issued to Dutcher.
Typically, some mechanism is provided for sealing the distal end of the lead against fluid entry. One common mechanism is to provide a seal which engages the fixation helix directly, as disclosed in the above cited Bisping patent. An alternative mechanism, particularly useful in the circumstance in which the helix is mounted to a shaft, either rotated by means of the conductor or by means of a screwdriver stylet is to operatively associate a seal with the helix shaft for preventing the ingress of body fluids, by mounting a washer shaped seal encircling the shaft. The seal may either rotate with the shaft, as disclosed in European Patent Application No. 049431 issued to Borghi et al or the shaft may rotate relative to the seal as disclosed in U.S. Pat. No. 5,259,394 issued to Bens. If the seal rotates with the shaft, the seal may be located between two washer shaped members located on the shaft as in the Borghi patent. If the shaft rotates relative to the seal, the structure may correspondingly be reversed, with the seal located between washer shaped members mounted in the electrode head as in U.S. Pat. No. 5,456,708 issued to Doan. Alternatively, the seal may simply be mounted to the shaft as in U.S. Pat. No. 5,514,173 issued to Rebell or to the interior of the electrode head by adhesive or other means as in the Bens patent and in U.S. Pat. No. 5,425,755 by Doan. Because that portion of the seal which rotates relative to the shaft or relative to the electrode head produces frictional resistance to rotation, it is in some cases desirable to configure this surface as a line contact to reduce friction, as is conventional in the art related to fluid seals associated with rotating shafts and as disclosed in the Borghi et al patent and in the '708 Doan patent.
In the event that it is desired to mount a seal located fixedly within the electrode head, surrounding the helix shaft, as in the Doan patents and the Bens patents cited above, it is necessary to pass the helix shaft through the seal. Similarly, if one wishes to employ washer-shaped members on either side of the seal as in the Bens and Doan '708 patents, the helix shaft must either carry the washer shaped members or must pass through the central apertures in the washer shaped members. This generally is not a problem, except in the context of a shaft which rotates relative to the seal when it is desired that the helix shaft be constructed with enlarged radius shoulders, located on either side of the seal to define advancement and retraction stops. If washer-shaped retainers are to be used as in the Doan '708 patent, the helix shaft must be fabricated out of two separate pieces.